Recently we have discovered that PPD-Tuberculin triggers DNA synthesis in spleen cells cultured from non-immunized animals. The responding cells were found to be bone-marrow derived (B) lymphocytes. Also PPD has been found to stimulate polyclonal antibody production in cultured splenic lymphocytes. Consequently, we wish to determine what role these phenomena play in the host infected with the tubercle bacillus thereby gaining further insight into the relationship of the major classes of lymphocyte (T and B) to hypersensitivity and immunity to tuberculosis. Studies also will be directed at characterizing the active principle(s) of PPD responsible for activation of B lymphocytes.